Users often employ various signal attenuation and conditioning methods in electric circuit design. Accurately measuring small values of differential voltages superimposed on much larger, varying common mode voltages requires a high value of common mode rejection ratio (CMRR) in the data acquisition system. When the input voltages exceed the system's analog power supply voltage, these voltages need to be attenuated by external resistors before being applied to differential processing inputs, such as an instrumentation amplifier or a differential-input analog-to-digital converter (ADC).
Designers may either purchase costly pre-trimmed instrumentation amplifier ICs or include their own trimming arrangements. Some form of adjustable resistor is used to deliberately unbalance the resistive input attenuator and trim the common mode rejection (CMR). Such resistors may be adjusted through physical means (i.e. a mechanical wiper moved with a screwdriver), or may be implemented as electronically programmable or electronically tunable component arrays.
In order to achieve the required CMRR, users often employ trimmable, high precision resistors. Such resistors are expensive and need to be calibrated. Furthermore, using, adjusting, and calibrating such resistors to achieve common mode performance is often unsuitable for an application because of cost, size, power, or other grounds.
Physically adjustable components require a manual calibration step on the production line. Electronically adjustable component arrays can facilitate an automated process but their value resolution, i.e. step size, is often marginal for the task. Continuously tunable resistances such as FETs and light-dependent resistors are generally not stable enough for use in many applications. All these approaches consume considerable additional circuit board space, which is sub-optimal when compact system design is required. Additionally, the costs associated with the aforementioned solutions are high.